This application claims the priority of German patent document 100 60 566.4, filed Dec. 1, 2000, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a friction body of silicon-infiltrated, carbon fiber-reinforced porous carbon, a method for the manufacture of such a friction body, and a use of such a friction body.
Friction bodies of carbon fiber-reinforced silicon carbide are known, for example, from DE 44 38 455 C1 (U.S. Pat. No. 6,086,814).
EP 0 797 555 A1 (U.S. Pat. No. 6,042,935) discloses a friction unit, which consists of a core body and at least one friction layer. For this unit, the friction layer is to be produced initially as a carbon body, which is then infiltrated with silicon. Between the friction layer and the core body, a connecting layer is disposed, which contains essentially silicon carbide. The friction layer and the core body are produced separately and are then connected with one another. In the case of an internally vented brake disk, the core body may, for example, also be constructed in two parts, with the cooling channels being formed in the connecting plane. There are two proposed methods for connecting the friction layer and the core body. The friction layer and core body either (1) are produced from a carbon material, assembled and infiltrated jointly with silicon, or (2) are infiltrated separately with silicon and subsequently connected to one another, in that silicon or preferably silicon carbide is brought into the plane separating the friction layer and the core body. With regard to the composition of the friction layer, it is disclosed that this is to be optimized with regard to its frictional behavior. For this purpose, it is proposed that additives, which increase or decrease the friction, be used. Boron nitrite or aluminum phosphate decreases friction. On the other hand, silicon carbide powder with a particle size of 0.3 to 3.0 xcexcm increases friction. In addition, carbon fibers may be provided in order to increase thermal conductivity. The proportion of fibers in the thickness direction should be 3% to 10%. An optimization of the strength of the friction layer is not disclosed.
Compared to this state of the art, the problem arises of protecting the friction layer against damage by outbreaking.
This problem is solved according to preferred embodiments of the present invention. It is proposed that carbon fibers, which are shorter than the carbon fibers of the core body, be provided in the friction layer.
The present invention is based on the realization that fiber lengths, which are necessary for strength reasons in the area of the core body, lead to extensive outbreaking in the friction layer. The reason for this lies in that a particle, that breaks out of the surface of the friction layer tends to pull out surrounding areas over the carbon fibers, which are tied in. On the other hand, if the friction layer consists of a pure ceramic and carbon fibers are not provided in the friction layer, outbreaking also occurs over a large surface, since the necessary strength of the friction layer is missing here. With the shorter carbon fibers, provided pursuant to the present invention, a high strength of the friction layer is achieved in an advantageous manner. Outbreaking is limited to small areas since, because of the decreased length of the carbon fibers, a region of the friction surface that is outbreaking can tear out only a small surrounding region.
In accordance with an advantageous embodiment of a device according to the present invention, the length of the carbon fibers in the friction layer is less than 50% of the length of the carbon fibers in the core body. A length of 1 mm to 5 mm for the carbon fibers in the friction layer has proven to be advantageous.
Pursuant to a method according to the present invention, different possibilities for producing a friction body with short fibers in the friction layer are proposed.
According to a first alternative, the green compacts for the friction layer and the core body are produced separately, glued together and subsequently pyrolyzed and infiltrated with silicon. According to a second alternative, the friction layer is introduced during the original shaping of the core body, in which a green compact for the friction layer is placed into the original mold for the core body and subsequently the material for the core body is filled into the original mold. The further production of the friction body then takes place in a known manner. The special advantage of this alternative lies in that no additional binder is required between the friction layer and the core body.
According to a third alternative, initially the material for the friction layer and, subsequently, the material for the core body are enveloped in the original mold for the core body. Depending on the development of the process, the material for the friction layer can also be pre-consolidated after it is filled into the mold. The further production of the friction body then takes place in a known manner. The special advantage of this method lies in that a molding process is no longer required for the friction layer, because the friction layer is produced together with the core body. Moreover, in the case of a brake disk produced in this manner, a break between the friction layer and the core body is no longer visible.
It is particularly advantageous to use such a friction body or a friction body produced in this manner in highly stressed vehicle parts, especially as a brake disk, as a friction lining or as a clutch plate. The use in conjunction with a two-part core body, such as an internally vented core body, can be carried out particularly easily with the above method, but is not a prerequisite for implementing the method shown.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the present invention when considered in conjunction with the accompanying drawings.